Parameters of reactive capacity and aerobic capacity in physically trained rats will be compared to those of sedentary rats at Test Sessions administered in 6 month intervals throughout their life span. Pubescent rats (90 days) will be operantly conditioned to release a lever as quickly as possible to a conditioned stimulus (CS; providing avoidance latencies), that precedes the unconditioned stimulus (UCS) of electric shock by intervals of 1.0, .4, .2, and .1 sec. At each Test Session the physically trained group will be compared to the sedentary group on latency and consistency of these avoidances; the escapes made in response to the UCS; retention of the task from the last Test Session; and, the acquisition rate to reactive capacity asymtote. At puberty and at each Test Session therafter, a random sample of 18 trained and 18 sedentary rats will be sacrificed to estimate such brain oxidative capacity changes that may parallel reactive capacity changes as: a) oxygen consumption and cytochrome oxidase activity of tissue homogenates and isolated mitochondria; b) oxidative capacity of cerebral motor cortex and basal ganglia slices as measured by 14CO2 produced from oxidation of 14C-glucose and 14C-B-hydroxybutyrate. The physical training effect will be verified by a) a work capacity test as measured by an endurance run to exhaustion; b) whole muscle oxidative capacity; and c) the concentration SDH and cytochrome oxidase as markers of oxidative capacity. A potential age-related decline of the animals' perceptual apparatus will be monitored at each Test Session by surface recorded EEG visual and auditory evoked potentials. It is hypothesized that although skeletal muscle aerobic capacity is drastically different in the trained and untrained groups by the 180 day Test Session, reactive capacity and brain oxidative capacity differences will not emerge until the animals have aged (Test Session at 540, 720 days.)